Fuel cells are known sources of electrical energy. As an individual fuel cell typically produces insufficient electrical energy for any practical application, fuel cells are typically combined to form a fuel cell stack. Bipolar plates are typically employed to conduct current from cell to cell, and to provide channels for gas flow into the stack.
In a typical fuel cell stack, a number of bipolar plates are stacked alternatively with diffusion media, in an anode-medium-cathode-medium-anode arrangement, and then placed under pressure to seal the interfaces. Thus, a bipolar plate may serve as an electrode for each of two adjoining fuel cells. The electrical output required from the fuel cell stack determines the number of cells, and therefore, the number of bipolar plates needed.
It should be understood that, the more conductive a bipolar plate is, the fewer cells will be required to produce a given power output. It should also be understood that, the more conductive a bipolar plate is, the less heat energy it will emit. To produce smaller, lower-cost fuel cells, methods for improving conductivity of bipolar plates are therefore desirable.
Contact elements are often fabricated from graphite, which is light-weight, corrosion-resistant, and electrically conductive. The electrical and thermal conductivity of graphite, however, is quite low compared with light weight metals such as stainless steel, aluminum, titanium, and their alloys. Unfortunately, such light weight metals are either not corrosion resistant in the fuel cell environment, and, therefore, contact elements made from such metals deteriorate rapidly, or they form highly electronically resistive oxide films on their surface that increase the internal resistance of the fuel cell and reduce its performance.
U.S. Pat. No. 5,624,769, which issued on Apr. 29, 1997, and reissued on Jul. 17, 2001, as reissue patent Re 37,284 (collectively “the 769 patent”), is assigned to General Motors. The disclosures of U.S. Pat. No. 5,624,769 and Re 37,284 are incorporated herein in their entireties.
The '769 patent, which is entitled “Corrosion resistant PEM fuel cell,” discloses a PEM fuel cell having electrical contact elements (including bipolar plates) comprising a titanium nitride coated light weight metal (e.g., Al or Ti) core, having a protective metal layer intermediate the core and the titanium nitride. The protective layer is susceptible to oxidation in the operating environment of the fuel cell so as to form a barrier to further corrosion at sites where the layer is exposed to such environment. Oxides formed on the protective metal layer have relatively low electrical resistivity so as not to substantially increase the internal resistance of the fuel cell.
An oxide layer, however, is native on exposed surfaces of the bipolar plate before any such protective layer is deposited on the plate. Oxide layer thickness is directly related to the potential drop across the interface of stainless steel, aluminum, and titanium (most oxides in fact). Reduction of these potential, and correlated IR, drops tend to improve fuel cell efficiency. Therefore, to improve conductivity, it is desirable to control (i.e., limit, reduce) the thickness of the oxide layer.
Currently, material to be used for the fabrication of bipolar plates (and, therefore, for the fabrication of fuel cells from such bipolar plates) is deoxidized electrochemically. Materials that have been deoxidized by electrochemical activation for use in the fabrication of bipolar plates are known to have ˜10 mV potential drop for the electrode couple at 1 A/cm2. Electrochemical activation, however, is known to take a relatively long time. In some cases, as much as 45 minutes is required to achieve the desired potential drop.